This invention relates to electric power systems, and particularly to power system overvoltage protection. The need arises to suppress power surges which occur when utilities switch reactive components such as capacitor banks.
Metal Oxide Varistors (MOVs) have been applied to limit overvoltages resulting from such power surges caused when switching capacitor bank locations. Significant improvements can be achieved by applying power electronics to MOVs to develop a Dynamic Voltage Suppressor (DVS) scheme. The DVS improvements include inrush overvoltage reduction, reduction in probability of restrike and ability to reenergize in a relatively fast time.
Electric utilities are facing an increasing dependence on the control of reactive compensation for normal and contingency operation. In many applications these reactive components are switched on a daily basis to match the cyclic demand of power. As used herein, the terminology xe2x80x9creactive componentsxe2x80x9d is meant to comprise capacitors, inductors, transformers and power lines.
Capacitor switching by traditional methods is resulting in a greater exposure to overvoltages as the number of reactive power sources increase. This is becoming a growing concern for power quality, equipment overvoltages and transient system stability. Switching of shunt capacitor banks primarily affects power quality. Overvoltages are caused by high inrush currents when closing or possible re-strikes immediately after opening. Methods to reduce overvoltages are continually being explored, such as controlled switching of breakers.
Transient stability would be enhanced by having the ability to switch a capacitor in or out without having to wait five minutes for the capacitor to discharge. Present methods of capacitor switching may be considered slow (traditional mechanically switched capacitor banks) or immediate (thyristor-based switched VAR capacitors). Shunt capacitor banks switched with traditional methods may have a cost range of $10-15/kVar but require at least a five-minute discharge time delay before re-energizing. Thyristor based switching would increase the cost to $30-40/kVar. This may be the only solution for applying reactive compensation if high transient voltages arise from line trip-out or a rapidly developing out-of-step condition. However for most transmission system applications, immediate or nearly instantaneous switching will not be required. For instance, the damping of slow dynamic voltage swings, or prevention of voltage collapse, requires a relatively fast response from between one-half to several seconds for controlling reactive compensation. A combination of traditional capacitor switching methods with DVS in a suitable configuration provides these response times so as to become an intermediate cost alternative.
MOVs, while being very good non-linear resistors for voltage surge limiting applications, suffer from the disadvantage that the suppression voltage level is about 1.7 times the normal operating voltage peak. Power electronics, especially thyristors, have proven to be useful in modifying their characteristics to make them more effective in accordance with this invention.
The DVS unit comprises an MOV with an anti-parallel thyristor pair across it as an integrated wafer or in separate blocks. When appropriately commanded, the thyristor pairs are fired to short out the MOV, as shown in FIG. 1. During steady-state operation, the thyristors are kept off, but are suitably triggered during periods when they need to modify the MOV arrester characteristics.
A single DVS unit may be applied for a short time (1 cycle) to suppress inrush overvoltages when energizing capacitor banks. When de-energizing, several DVS units are used to remove the trapped charge on the capacitor to allow relatively fast reclosing.